apparently a fairly rare piece of webbing….

from kakeeweb:
The Pattern 1925 Strap, supporting, rucksack was not listed in A.M.W.O. 793/1927, being considered a part of the Rucksack. Recognising that it was loose, by January 1942, it was separately catalogued in Section 23 (Accoutrements) of Air Publication 1086 Priced Vocabulary of Royal Air Force Equipment. It is 3/4-inches wide and approximately 15-inches long, with a brass tip at one end and a flat brass hook at the other. When the Rucksack, top part, is used alone, this Strap connects to a buckle at the bottom of the Rucksack. The hook is inserted into the D-shaped loop at the side of the Haversack hook on the right-hand shoulder strap of the Rucksack, top part.
When both parts of the Rucksack are worn, the strap is moved to a buckle on the bottom of the Rucksack, bottom part.

Perhaps because it is small and easily lost, or for some other unknown reason, this Strap is almost always missing - to the extent that there has been serious discussion among collectors suggesting that it was never actually on general issue at all. To some degree, that may be understandable wishful thinking, but it’s incorrect. The ease with which the Supporting strap could be lost seems to have recognized at some point. Rucksacks, top part, with a fixed Strap, as with Patt. ’37 Haversack straps, have been noted from a few manufacturers.

Click to enlarge the picture